The ecological water requirement of vegetation is a hotspot in the research of optimal allocation and ecological environment, and the indirect calculation method is the main method to quantify the ecological water requirement of natural vegetation in arid area. In order to calculate more accurate, by combining the 0.618 method and the Extremum method to improve the calculation of the submersible evaporation of the indirect method, we established the second order Gaussian model of vegetation coefficient K and forecasted the development trend of the vegetation area S with the grey model GM (1,1). The results showed that the optimized phreatic water evaporation-buried depth curve fitted the measured data better, and the vegetation coefficient were more accurate. Taking the Minqin County of Gansu Province as an example, the difference of area between recorded and calculated data in 2015 was small, and the results of ecological water requirement were not significantly different from those of FOA recommended Penman-monteith model. Based on which, the total ecological water requirement of Minqin County natural vegetation in the 2020 was predicted to be 40.608 million m3, of which the ecological water requirement of natural forest was 31.001 million m3, and the ecological water requirement was 9.607 million m3. The indirect calculation method is more accurate for the quantification of ecological water requirement of vegetation, which lays a foundation for the allocation and policy of ecological restoration in arid areas.